1. Field of the Invention
This invention relates to a device for crucible-free zone melting on crystalline rods, in particular semiconductor rods, in which the molten zone is produced in the rod which is to be treated by the electromagnetic field of an induction heating coil which surrounds the rod (the rod being supported at its ends) in annular fashion and at a distance, in which the induction heating coil is supplemented by a capacitor connected in parallel to the latter, to form an electric oscillatory heating circuit, and the oscillatory heating circuit is fed via a high-frequency line and at least one adjustable coupling element by a high frequency generator which supplies an a.c. voltage of adjustable frequency.
2. Description of the Prior Art
Such a device is described in the British Letters Patent No. 1,144,907. The high frequency generator of the arrangement is provided with a constantly variable output frequency and possesses an electric oscillatory circuit (also referred to herein as a tank circuit) in the output, having an inductance which is provided with a displaceable tapping which is connected to the oscillatory heating circuit via a coupling inductance or a coupling capacitor. The electrical power provided to the oscillatory heating circuit, and via the latter to the rod which is to be zone melted, is preferably set via the frequency of the a.c. voltage emitted from the high frequency generator. This a.c. voltage is selected to be such that in normal operation it differs relatively little from the characteristic frequency of the oscillatory heating circuit. Therefore, it is ensured that in view of the properties of the transmission elements employed, an optimum energy transfer to the crystalline rod which is to be treated, in particular the silicon rod, is achieved when the amplitude of the a.c. voltage occurring in the induction heating coil differs from the amplitude in the case of resonance by approximately 10 percent.
A further possibility of influencing the electric active power transmitted to the oscillatory heating circuit and to the rod which is to be zone melted is that of adjusting the variable coupling of the oscillatory heating circuit to the output of the high frequency generator.
The setting of the generator frequency and the coupling of the oscillatory heating circuit to the output of the high frequency generator are matters of experiment which must be made for specific rod diameters of a certain crystalline material, e.g. silicon. One thus easily achieves the aim of providing crucible-free zone melting of homogeneous rods. If, however, crucible-free zone melting is used for the treatment of rods having inhomogeneous diameters and/or inhomogeneous material properties, the prior art suggests that the only course that can be taken is to vary the setting of the generator output frequency and/or of the degree of coupling during the zone melting. However, this technique is not favorable in respect of the mechanically labile state of the molten zone, as undesired changes in the molten zone easily can occur, or the molten zone can even start to drip.
In particular, in the zone melting of silicon rods whose diameters are greater than 50 mm. the load zone, which must pass over the melting of the monocrystalline seed (which is required for the monocrystalline growth of the silicon crystallizing out of the molten zone) to the rod zone which is to be connected to the seed via a bottle neck and adjoining cone, and which has a normal, i.e. a considerably larger rod diameter, is very large. This might result in the aim of discovering generator frequency settings and/or coupling settings which can be used for a series of transition stages of the molten zone which can be used without damaging the molten zone, which in this state is particularly sensitive.